1. Field of the Invention
The present invention relates to a high-speed searching apparatus for a compact disc player (hereinafter, referred to as a CDP), and more particularly to a high-speed searching apparatus for controlling a searching operation of a pickup by using the moving distance of an optical pickup which is measured by an encoder mounted to a rotary shaft of a slide motor.
2. Discussion of the Related Art
A conventional high-speed searching apparatus for a CDP includes, as illustrated in FIG. 1, an RF amplifier 2 which amplifies the signals output from optical detectors (A-D) of an optical pickup 1, a low-pass filter 3 through which a lowbandwidth signal passes from the output signals of the RF amplifier 2, bandwidth signal passes, a waveform shaping circuit 4 which shapes the output signal of the low-pass filter 3, a track counter 5 which counts the output signal of the waveform shaping circuit 4 and applies the counted signal to a main processing unit (MPU), an E-F amplifier (6) which amplifies the track error signals output from optical detectors (E-F) of the optical pickup 1, a tracking servo circuit 6 and a tracking drive circuit 7 which generates a drive signal for tracking operation by using the output signal of the E-F amplifier 6, and a slide servo circuit 9 and a slide drive circuit 10 which generate a slide motor drive signal by using a direct current value of the tracking drive circuit 8.
In the drawing, the current sources I.sub.TF, I.sub.TR, I.sub.SF and I.sub.SR are connected to a positive power source VCC. A negative power source VEE supplies the currents for tracking normal direction jump, tracking inverse direction jump, slide normal direction movement and slide inverse direction movement. The currents therefrom are controlled by switches TS1, TS2, SS1 and SS2, respectively.
The reference numeral 11, in FIG. 1, represents a slide motor, numeral 12 represents a worm gear which moves to the left and right according to the rotation of the slide motor 11, and numeral 13 represents a disc.
In FIG. 2, the RF amplifier 2 and the E-F amplifier 6 are illustrated in detail. As shown in FIG. 1, the RF amplifier 2 sums up and amplifies the signals detected at the optical detectors A-D of the optical pickup 1, and the E-F amplifier 6 subtracts and amplifies the signals detected at the optical detectors E,F of the optical pickup 1.
In the above system, the optical detectors E-F of the optical pickup 1, which are necessary parts for the optical pickup apparatus utilizing a three-beam method, are adapted to detect the track error. The optical detectors A-D are adapted to detect a main beam reflected from a disc, and the total magnitude of the signals detected at the L four optical detectors A-D determines whether or not a pit exists on a disc.
FIGS. 3A to 3C illustrate waveforms for each portion of FIG. 1 where the optical pickup 1 crosses the tracks on the disc 13. FIG. 3A illustrates the output waveforms of the RF amplifier 2, wherein the envelopes represent the waveforms generated when the optical pickup 1 passes over the tracks, and the high-frequency components within the envelopes result from the pits on the tracks. FIG. 3B illustrates the waveforms from the RF amplifier 2 which are filtered by the low-pass filter 3, and FIG. 3C illustrates the waveforms from the low-pass filter 3 which are shaped by the waveform shaping circuit 4. In the above system, the waveforms output from the waveform shaping circuit 4 are input to a clock terminal of the track counter 5, so that the number of the tracks on which the optical pickup 1 crosses can be counted by the track counter 5.
Generally on a disc 13, a continuous spiral track is formed from the inner periphery to the outer periphery thereof, and the optical pickup 1 moves along the spiral track of the disc 13 at a predetermined linear speed. An absolute time from the predetermined point of the disc 13 to the utmost inner periphery point, it is possible to calculate where a point resides on the track.
The equation for calculating the point is as follows: ##EQU1## Where, r.sub.0 is a radius of the utmost inner periphery of the portion that information is recorded,
Vo is a linear speed, PA1 P is a track pitch, PA1 t is an absolute time, and PA1 r is a radius of an abitrary point.
Assuming that, for example, Vo=1.2 m/sec, P=1.6 .mu.m, and r.sub.o =25 mm, ##EQU2## the distance r between two points r.sub.1, and r.sub.2 in accordance with the radius of the disc becomes, EQU r=r.sub.2 -r.sub.1, and
the number of tracks N is represented by ##EQU3##
Therefore, it is possible to calculate the number of tracks; ##EQU4##
As a result, the number of the track up to a point that the absolute time lapses from the utmost inner periphery where information is recorded can be calculated by the above mentioned equation.
The searching operation of such a conventional searching apparatus as illustrated in FIG. 1 will be explained, with reference to the flow chart of FIG. 4. Firstly, the number of the track at a point from the utmost inner periphery to the present pickup position is calculated with the absolute time thereof (S1), and the number of the track at a point to be searched is calculated with the absolute time thereof (S2). Then the number of the track to be jumped is calculated by subtracting the former value from the latter value (S3). Thereafter, the switch SS1 is turned on to apply the slide normal direction movement current I.sub.SF to the slide drive circuit 10 so that the slide is moved in the normal direction (S4). At this moment, the MPU counts the output of the track counter 5 (S5), and checks continuously whether the slide reaches the half point of the track to be searched (S6). In case that the slide reaches the half point, the switch SS1 is turned off and the switch SS2 is turned on so that the slide reverse direction movement current I.sub.SR for the braking operation is applied to the slide drive circuit 10 (S7). After this occurs, the slide moves with a reduced speed towards the track of the point to be searched (S8), and then the operations as above are completed.
FIG. 5 illustrates the searching operation as explained above in detail.
However, in such a conventional searching apparatus, there has been some problems in the that, in case of a high-speed searching operation, the envelope frequency appearing across the track becomes high. Since the magnitude of the RF component within the envelope due to a pit on the track has a fixed value of 3T-11T (where, T is 1/2 of a channel clock period, and the channel clock frequency is 4.3218 MHz), the RF component has a frequency of 196 KHz-720 KHz. Accordingly, if the speed of the optical pickup moving across the track is excessively high, the signal of 196 KHz, which is the lowest frequency among the RF signals, cannot be satisfactorily eliminated. In general, the limitation for satisfactorily eliminating the lowest frequency is about 1/10 of the lowest frequency among the RF signals, and the upper limitation of the waveform which is input to the track counter 5 is about 20 KHz.
A conventional disc has about 20,000 tracks therein, and about 1 second is required for searching from the utmost inner periphery to the utmost outer periphery of the disc. Thereby it is difficult to execute a high-speed searching operation.